LV generator protection - engineering site - Schneider Electric

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3.1.2. Short-circuit current protection3.1.2.1. Making the short-circuit currentThe short-circuit current is the sum:b of an aperiodic currentb of a damped sinusoidal current.The short-circuit current equation shows that it is made according to threephases.E79365EI rms 1subtransientconditions2transientconditions3steady stateconditionsgenerator withcompoundexcitation orover-excitationgenerator withserial exitationfaultappears10 to 20 ms0.1 to 0.3 sFigure 14: Short-circuit current level during the 3 phases.Subtransient phaseWhen a short-circuit appears at the terminals of a generator, the current is firstmade at a relatively high value of around 6 to 12 ln during the first cycle(0 to 20 milliseconds).The amplitude of the short-circuit output current is defined by three parameters:b the subtransient reactance of the generatorb the level of excitation prior to the time of the fault andb the impedance of the faulty circuit.The short-circuit impedance of the generator to be considered is the subtransientreactance expressed as a % of Uo (phase-to-neutral voltage) by themanufacturer x”d. The typical value is 10 to 15 %.We determine the subtransient short-circuit impedance of the generator:UX"d2 n x"d= where S = 3US 100NI N.Transient phaseThe transient phase is placed 100 to 500 ms after the time of the fault. Startingfrom the value of the fault current of the subtransient period, the current drops to1.5 to 2 times the current ln.The short-circuit impedance to be considered for this period is the transientreactance expressed as a % Uo by the manufacturer x'd. The typical value is 20to 30 %.Steady state phaseThe steady state occurs above 500 ms.When the fault persists, Set output voltage collapses and the exciter regulationseeks to raise this output voltage. The result is a stabilised sustained short-circuitcurrent:b if generator excitation does not increase during a short-circuit (no field overexcitation)but is maintained at the level preceding the fault, the current stabilisesat a value that is given by the synchronous reactance Xd of the generator. Thetypical value of xd is greater than 200 %. Consequently, the final current will beless than the full-load current of the generator, normally around 0.5 ln.b If the generator is equipped with maximum field excitation (field overriding) orwith compound excitation, the excitation “surge” voltage will cause the faultcurrent to increase for 10 seconds, normally to 2 to 3 times the full-load currentof the generator.T (s)17
Protection and Monitoringof a LV Generator Set3.1.2.2. Calculating the short-circuit currentManufacturers normally specify the impedance values and time constantsrequired for analysis of operation in transient or steady state conditions.Impedance table: Leroy Somer generator(kVA) 75 200 400 800 1600 2500x"d (%) 10.5 10.4 12.9 10.5 18.8 19.1x'd (%) 21 15.6 19.4 18 33.8 30.2x'd (%) 280 291 358 280 404 292Resistances are always negligible compared with reactances.The parameters for the short-circuit current study are:Value of the short-circuit current at generator terminalsShort-circuit current strength in transient conditions is:sorsU Nis the generator output phase-to-phase voltage (Main source).Note: this value can be compared with the short-circuit current at the terminalsof a transformer. Thus, for the same power, currents in event of a short-circuitclose to a generator will be 5 to 6 times weaker than those that may occur with atransformer (main source).This difference is accentuated further still by the fact that generator set power isnormally less than that of the transformer.ExampleE79474EMVGSLVNCNCMain/standbyNon-priority circuitsPriority circuitsNC: normally closed.NO: normally open.Figure 15.When the LV network is supplied by the Main source 1 of 2000 kA, the shortcircuitcurrent is 42 kA at the main LV board busbar. When the LV network issupplied by the Replacement Source 2 of 500 kVA with transient reactance of30 %, the short-circuit current is made at approx. 2.5 kA, i.e. at a value 16 timesweaker than with the Main source.18
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